Slot port vs Round port confusion!!!
11-18-2008, 03:24 PM
#21
Administrator
Join Date: Feb 2006
Posts: 16,855
^ nobody was really asking how a vent operates though.. 
Your theory appears based on air speed, which is incorrect.
This is true.
Where your theory failsthough is trying to relate tuning to air speed. If you place a 4" woofer with 1mm Xmax in that 1cuft box will it be tuned to 35hz? yes. If you place a 10" woofer with 25mm Xmax in that 1cuft box will it be tuned to 35hz? yes. Will the relative air speed in the vent be the same for the 4" as the 10"? nope. Why? Because the 10 is moving in excess of 150 times the air per stroke than the 4" and displacement dictates air speed. This is key in determining the area of the vent, but not the length, ie tuning.
Consider this 10" woofer playing a single note - the cone moves more or less depending on amplitude. So when at low volume and cone displacement the air speed in the vent will be much less than at high volume and cone displacement, yet the tuning stays the same (if the vent is sized properly). The frequency is not changing but the air speed is because air speed is related to displacement. The tuning of the vent is unchanged though.
A higher tuned vent does need to have more area though as the relative airflow through it IS faster from a design standpoint. How come? It is due to the total displacement of air over time. At resonance, a 30hz tuned vent has to move XX air 30times per second. At resonance, a 45hz tuned vent has to move XX air 45times per second. The theoretical total displacement over time for the 45hz vent will be 50% more than the 30hz vent and therefore it needs to have more area as a vent is always tuned based on its resonance.
Remember though that the air speed in either vent is dictated by the frequency being played by the driver and the excursion of the cone - it has nothing to do with the length of the vent. Whether that vent is 13" long or 7" long, it will have to move 45 excursions of air per second if 45hz is being played. All the length of the vent does is tailor how that sounds to our ear and how well the driver reacts to it.
Your theory appears based on air speed, which is incorrect.
A 1cuft box tuned to 35hz takes a 3 inch port that is 13.36 inches long.
Where your theory failsthough is trying to relate tuning to air speed. If you place a 4" woofer with 1mm Xmax in that 1cuft box will it be tuned to 35hz? yes. If you place a 10" woofer with 25mm Xmax in that 1cuft box will it be tuned to 35hz? yes. Will the relative air speed in the vent be the same for the 4" as the 10"? nope. Why? Because the 10 is moving in excess of 150 times the air per stroke than the 4" and displacement dictates air speed. This is key in determining the area of the vent, but not the length, ie tuning.
Consider this 10" woofer playing a single note - the cone moves more or less depending on amplitude. So when at low volume and cone displacement the air speed in the vent will be much less than at high volume and cone displacement, yet the tuning stays the same (if the vent is sized properly). The frequency is not changing but the air speed is because air speed is related to displacement. The tuning of the vent is unchanged though.
A higher tuned vent does need to have more area though as the relative airflow through it IS faster from a design standpoint. How come? It is due to the total displacement of air over time. At resonance, a 30hz tuned vent has to move XX air 30times per second. At resonance, a 45hz tuned vent has to move XX air 45times per second. The theoretical total displacement over time for the 45hz vent will be 50% more than the 30hz vent and therefore it needs to have more area as a vent is always tuned based on its resonance.
Remember though that the air speed in either vent is dictated by the frequency being played by the driver and the excursion of the cone - it has nothing to do with the length of the vent. Whether that vent is 13" long or 7" long, it will have to move 45 excursions of air per second if 45hz is being played. All the length of the vent does is tailor how that sounds to our ear and how well the driver reacts to it.
11-24-2008, 11:24 PM
#22
500 Watt CAFz'r
Join Date: Apr 2006
Posts: 633
Just my two cents... round aeroports are great, if you have them of course... otherwise I would personally build a slot port box. I built a small slotport box for some older 12's and ended up rebaffling the face and dropping my friends rockford 10's into it. Considering the low amount of power this box really did sound amazing, for what it was. I have never made the slots thinner than 1.5 inches (i use scrap pieces of 3/4 mdf while constructing) and always use a router/sander to roundover the corners.
11-25-2008, 01:16 AM
#23
Well I'm no expert with enclosures, but when it comes to physics I'm not so shabby, so I think I can explain adequately enough.
We have an enclosure 1 cubic foot in size, a 10 inch driver, and a port. First things first, length is relative only to the port in question. Chances are that the displacement of air within all the port designs is the same, which makes sense. The function of a port is to allow air flow. Based on tuning frequency, the amount of air being displaced and allowed to flow changes, since the air within the port itself is considered neutral, and moves either inward or outward depending on the excursion and incursion of the driver being used. Higher frequency requires more rapid airflow, thus would require a smaller port than a low frequency tune in the same enclosure. Here's an example.
A 1 cubic foot box tuned to 35 Hz takes a 3 inch port that is 13.36 inches long. The same box tuned to 45 Hz takes a port that is 7.21 inches long. Again, the higher frequency requires air to be moved more rapidly, and thus the relative amount of "neutral" air must be reduced. The driver can obviously move the air in a shorter port much faster than it can with a longer one.
I realize that it doesn't really make sense to a lot of people at first, but basically what I just said is the way it works. It all comes down to the box and the amount of "neutral" air within the port itself, which determine the tuning frequency.
We have an enclosure 1 cubic foot in size, a 10 inch driver, and a port. First things first, length is relative only to the port in question. Chances are that the displacement of air within all the port designs is the same, which makes sense. The function of a port is to allow air flow. Based on tuning frequency, the amount of air being displaced and allowed to flow changes, since the air within the port itself is considered neutral, and moves either inward or outward depending on the excursion and incursion of the driver being used. Higher frequency requires more rapid airflow, thus would require a smaller port than a low frequency tune in the same enclosure. Here's an example.
A 1 cubic foot box tuned to 35 Hz takes a 3 inch port that is 13.36 inches long. The same box tuned to 45 Hz takes a port that is 7.21 inches long. Again, the higher frequency requires air to be moved more rapidly, and thus the relative amount of "neutral" air must be reduced. The driver can obviously move the air in a shorter port much faster than it can with a longer one.
I realize that it doesn't really make sense to a lot of people at first, but basically what I just said is the way it works. It all comes down to the box and the amount of "neutral" air within the port itself, which determine the tuning frequency.
Im not gonna get into the nitty gritty of all this, but basically what Dukk said about total air displacement over time is how that goes.
Combined with some other key factors, this is very critical in SPL applications.
Last edited by Father Yuli; 11-25-2008 at 01:20 AM.
11-25-2008, 11:11 AM
#24
50 Watt CAFz'r
Join Date: May 2007
Posts: 384
to paraphrase Dukk, Moving a lot of air through a small hole in a small amount of time equals a noisy port. The bigger the hole(or port) the less noise, however it must be tuned properly. Ports are designed to get maxium out put at a certain range of Hz and peak at a certain Hz.The size and length of the port is determined by the volume of the box, the sub and the desired tuning frequency.
11-25-2008, 11:35 AM
#25
Tuning cares not for which driver is used. If you use any box design program, you'll notice that the port volume is exactly the same for a given tuning frequency and box volume. Example: a 1 cube vented box tuned to 30 hz gives the exact same port dimensions whether a 10" driver from Alpine is used or an 18" driver from ACI. The box is tuned, not the driver. It's up to the system designer to determine what he/she wants in conjunction with the driver's abilities, and design parameters. Obviously, some designs are much better than others, and it's up to the system designer to take into account air velocity in the ports, cone excursion, group delay, etc...to meet the design criteria.
Last edited by veeman; 11-25-2008 at 11:37 AM.
11-25-2008, 11:47 AM
#26
50 Watt CAFz'r
Join Date: May 2007
Posts: 384
That's what I meant when I included the sub in what determines the port. yes the box is tuned but you have to take everything in account to do it properly.To the inexperieced person, if you tell them that the box it self is tuned it implies that any sub will perform well in any tuned box.That is why we use theile small parameters so that we know what that perticular sub will do in that perticular box at the desired frequency range! Sorry if I muddled it all togerher.
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